This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-262328, filed Sep. 16, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a photonic crystal used in a light functional element, a multi-layer wiring board and a steric wiring board indispensable for a high density mounting in a portable equipment, and a method of manufacturing the same.
A three dimensional structure having a structure of several xcexcm to hundreds of xcexcm can be applied to a photonic crystal used in a light functional element such as a branching filter, an optical waveguide, a light delay element or a laser and to a steric wiring used in, for example, a built-up wiring board. In the photonic crystal, a regular periodic structure is required to be formed of substances differing from each other in the refractive index.
A photonic crystal is greatly featured in its optical characteristics. For example, it is possible to produce a wavelength region called photonic band gap, in which the light is not transmitted in any direction (E. Yablonovitch, Phys. Rev. Lett. 58 (20), 2059 (1987)). Also, the photonic crystal exhibits very large optical anisotropy or dispersibility. Therefore, an optical waveguide, a polarizer and a branching filter, which permit controlling the natural light emission and have a very small radius of curvature of a curved corner, have been proposed to date, and it is highly expected for these apparatuses to be put to a practical use.
A three dimensional structure having a distribution of refractive index such as a photonic crystal can be prepared by, for example, a method of laminating beads of silica or a polymer, a method using a self-organized structure such as a polymer, a self-Croning process utilizing a CVD process, a method of three dimensionally dry-etching a semiconductor in three directions, a method of laminating wafers, a method of laminating polycrystalline silicon (polysilicon) layers, a method of forming a distribution of composition of photosensitive agent photopolymerizing a medium consisting of two kinds of photosensitive agents, and a light-shaping process for three dimensionally photo-setting a polymerizable monomer. In each of these methods, the shape that can be formed is limited. For example, in the method of laminating beads or in the method of using a self-organized structure such as a polymer, the three dimensional structure that can be formed is limited. In the case of the self-Croning process, the dry etching method, the wafer fusing method or the polysilicon layer laminating method, a relatively costly semiconductor process is required. In addition, the width of the material selection is narrow. In the method of forming a composition distribution, it is difficult to obtain a large refractive index ratio among regions differing from each other in the refractive index, with the result that the materials used are limited to polymer materials. Similarly, the materials used are limited to polymer materials in the light-shaping process. Also, it is impossible to form isolated regions such as detached territories, making it necessary to form all the regions continuously.
On the other hand, the steric wiring is indispensable to a high density mounting, and various methods are proposed for formation of the steric wiring. In general, these steric wirings are multi-layered structure such as a built-up wiring board prepared by laminating two dimensional printed wiring boards and a multi-layered wiring board. It is difficult to form a steric wiring having a free three dimensional shape. The built-up wiring board or the multi-layered wiring board has a structure that adjacent wiring layers are connected to each other by a conductive column called via. The via is formed by processing a coated insulating layer by a photolithography process using a photosensitive polyimide or resist. For forming a via by such a method, it is necessary to repeat a plurality of times the steps of resist coating, light exposure and etching, making the via formation highly laborious. In addition, it is difficult to improve the yield.
It is also possible to form the via by forming a through-hole (via hole) of a predetermined size in an insulating substrate constituting a printed wiring board by using a drill or a CO2 laser, followed by applying plating to the via hole or by filling the via hole with a conductive paste. In these methods, however, it is difficult to form freely a fine via having a size of scores of microns or less at a desired position.
As a method of forming a conductive column without forming a via hole in an insulating substrate, proposed is an anisotropic conductive film prepared by forming a conductive column by an electroless plating in a thickness direction of a three dimensional porous film such as PTFE as disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 55-161306, Japanese Patent Disclosure No. 7-207450, U.S. Pat. No. 5,498,467 and Japanese Patent Disclosure No. 11-25755. In this method, it is possible to form a conductive column extending in the thickness direction of the film without forming a via hole in a predetermined position.
Where the anisotropic conductive film having a conductive column formed therein is used in the via layer of a multi-layered wiring board, not as a single layer anisotropic conductive film, it is necessary to ensure a good electrical and mechanical bonding properties between the via end face and a pat of the wiring, good bonding properties with the wiring layer, in which a wiring is formed, in the insulating layer portion, a high mechanical strength of the insulating layer, and good electrical insulating properties. However, since an open pore is formed within the film, the decrease in the insulating properties caused by the surface conduction on the inner wall of the pore, which is derived from the moisture absorption, is brought about as a problem in the case of using the film in the via layer (i.e., an insulating substrate having a via formed therein) of the multi-layered wiring substrate. Also, it is impossible to ensure a sufficiently high mechanical strength. In the conventional technology pointed out above, the porous film is impregnated after the via formation with, for example, a thermosetting resin to make the porous portion solid so as to ensure the required adhesivity. However, if the resin is impregnated after the via formation, the end face of the via is covered with the thermosetting resin, leading to a poor connection and an increased contact resistance. For removing the resin layer covering the end face of the via, a troublesome extra step is required.
Also proposed is a method of forming an anisotropic conductive sheet in which the via portion alone is porous and the portion other than the via portion is in the form of a solid film. In this method, a predetermined region of a polysilane sheet is exposed so as to bring about photo-oxidation and, thus, to convert the exposed portion into polysiloxane, thereby making the sheet porous. Then, the porous portion is loaded with a conductive substance by, for example, a plating technology so as to form a conductive column. In this technology, the insulating material used is limited to polysilane. It should be noted that polysilane tends to be deteriorated by an acidic substance, by an oxidizing reaction, etc. Also, polysilane does not exhibit an adhesivity in general. Therefore, in the case of using an anisotropic conductive sheet formed of polysilane as a via layer of a multi-layered board, an adhesive layer for bonding adjacent layers is newly required. What should be noted is that the end face of the via tends to be covered with the new adhesive layer.
A porous film prepared by elongating a uniform film such as PTFE is used as a three dimensional porous film in which a wiring or a via is formed. Particularly, where the film is made porous by elongation, an irregular three dimensional structure including relatively large knot-like structures and a fibrous structure mutually connecting these knot-like structures tends to be formed. The porous film consisting of such an irregular three dimensional structure tends to be nonuniformly shrunk within a film plane by the change in temperature and by the dipping in a solvent, making it difficult to maintain a high dimensional accuracy. Also, even if a conductive pattern is formed by exposure, the exposing ray is scattered by the nonuniform structure, making it difficult to form a satisfactory exposure pattern. Further, in the conductive column formed in such a porous film, a continuous phase of the conductive material within the pore is formed in an irregular shape, resulting in failure to obtain a sufficient conductance. In order to allow the conductive column to exhibit a satisfactory conductivity, it is necessary for the conductive material such as copper, which is loaded in the pore, to be loaded within the pore continuously with a high loading rate. For achieving such a loading state, it is necessary for the pores of the three dimensional porous film to have a regular shape and to be uniform in the pore diameter. It is also necessary for the three dimensional porous film not to include an obstacle structure such as a knot-like structure and for the pores to be formed homogeneously within the film.
In general, a multi-layered board is prepared by forming a conductive layer consisting of, for example, copper on each surface of an insulating sheet having a via formed therein, followed by patterning the conductive layer to obtain a double-sided wiring board having an electric circuit formed on each surface and subsequently laminating a plurality of such double-sided wiring boards. In this case, however, the surface of the insulating sheet is made irregular because the wiring portion having a certain thickness is selectively formed on the surface of the insulating sheet. The irregularity formed between the wiring portion and the non-wiring portion gives rise to a problem when the double-sided wiring boards are laminated one upon the other. The problem is serious particular where the insulating layer is formed thin, making it necessary to flatten the irregularity by some means. As a means for flattening the irregularity, a prepreg impregnated with a thermosetting resin is used in general as an insulating sheet, and the thermosetting resin is loaded in the irregularity in the laminating step. In this method, however, it is necessary to make the insulating sheet sufficiently thicker than the wiring layer so as to make it difficult to decrease sufficiently the thickness of the insulating sheet. Where the insulating sheet is thick, it is impossible to make the aspect ratio of the via (i.e., a ratio of the height to the diameter of the via) sufficiently large and, thus, the via diameter cannot be diminished. It follows that it is also impossible to diminish the wiring pitch and, thus, it is difficult to form a fine wiring pattern.
Further, where a multi-layered board is prepared, a conductive layer made of, for example, copper is formed first, on a via layer having a via. Then, the conductive layer is patterned to form a wiring and, thus, a via layer and a wiring layer, followed by laminating the wiring layers so as to prepare a multi-layered board. What should be noted is that required is a successive process such that a circuit is formed after formation of the via layer. It is impossible to collectively laminate wiring layers and via layers formed separately because it is very difficult to form independently a wiring layer without using an insulating substrate requiring a via formation and to hold and transfer the independent wiring layer for lamination.
A technique for overcoming the above-noted problem is disclosed in Japanese Patent Disclosure No. 10-321989. Specifically, it is disclosed that a circuit is formed on a honeycomb-like porous sheet such as a mesh-like screen or a punching sheet such that the screen is enclosed by the circuit. According to this technique, the circuit is held by the mesh, and the conductive portion of the circuit is exposed to the outside, making it possible to form a multi-layered structure as it is. However, since the screen is in the form of a plain weave of a fiber, it is relatively difficult to ensure the stability of the shape and the size. Also, since it is necessary to increase the fiber diameter to scores of xcexcm in order to ensure a sufficient mechanical strength, the technique disclosed in the prior art quoted above is not adapted for formation of a fine circuit wiring not larger than about scores of quoted above is not adapted for formation of a fine circuit wiring not larger than about scores of xcexcm. Further, in the case of using a honeycomb-like sheet, it is necessary to form a conductive layer on each of the upper and lower surfaces of the sheet in order to achieve conduction in the lateral direction. The formation of the conductive layer naturally gives rise to irregularity. In order to eliminate the irregularity of the wiring, it is necessary to form an insulating layer substantially equal in thickness to the wiring in the non-conductive portion, too. Also proposed in, for example, Japanese Patent Disclosure No. 55-161306 is a technique of forming a striped conductive region in a three dimensional porous film. However, the laminate structure of such a three dimensional porous film is not disclosed nor suggested in the prior art.
In the method of preparing a multi-layered wiring board by laminating a plurality of sheets each having a wiring pattern or a via formed therein, the positional deviation in the laminating step gives rise to a problem. The tendency is rendered prominent as the fineness of the wiring proceeds. Particularly, in the case of laminating the three dimensional porous film described above, the position aligning accuracy is not high because the three dimensional porous film is inferior to an ordinary solid film in the dimensional accuracy, mechanical strength, etc.
As described above, in forming a three dimensional structure having the refractive index distributed in a three dimensional direction such as a photonic crystal, the conventional method was defective in that the shape formed is limited and that it was impossible to obtain a large difference in the refractive index. Also, the width of selection of the materials was small in the conventional method.
Where a multi-layered wiring board is used in forming a steric wiring, troublesome steps are required in laminating sheets each having a wiring formed therein or in forming a via. Also, in the case of using a sheet having a conductive column formed in a porous film, the mechanical strength and electric insulating properties tend to be lowered because the sheet is porous. It is certainly possible to improve the mechanical strength and the electrical insulating properties by impregnating the porous sheet having a conductive column formed therein with resin. In this case, however, the end face of the conductive column is covered with the impregnated resin so as to lower the electrical characteristics. Further, it is difficult to decrease the thickness of the wiring board because it is necessary to flatten the irregularity of the wiring in forming the multi-layered board. Therefore, it was impossible to diminish the via diameter, resulting in failure to form a fine wiring. Also, the manufacturing process is rendered troublesome because it is necessary to form a circuit on an insulating sheet having a via formed therein. Where a multi-layered wiring board is prepared by laminating a plurality of sheets, it is difficult to align the sheets accurately, giving rise to the problem that it is difficult to make the wiring and the via fine.
As described above, it was difficult to manufacture easily with a low cost a photonic crystal having the crystal structure changed in various fashions by the combination of substances sufficiently differing from each other in the refractive index. Also, in the multi-layered wiring board, it was impossible to form a fine via freely and with a high accuracy and to form in the board in a desired pattern a steric wiring arranged in a three dimensional direction.
An object of the present invention is to provide a three dimensional structure having a three dimensional refractive index, wherein the three dimensional structure has a large difference of refractive index.
Another object of the present invention is to provide a three dimensional structure used suitably as a multi-layered wiring board or a steric wiring board having a high degree of freedom in the circuit design and having a fine wiring.
Another object of the present invention is to provide a three dimensional structure having a fine wiring and a via and excellent in electrical characteristics.
Further, still another object of the present invention is to provide a method of manufacturing the particular three dimensional structure.
According to a first aspect of the present invention, there is provided a three dimensional structure, comprising: a porous body; and a plurality of regions formed in the porous body and loaded with a substance; wherein an average period of a part of said plural regions loaded with said substance is 0.1 to 2 xcexcm to form a photonic band.
In the three dimensional structure of the present invention, the region for forming the photonic band is supported by a porous body, making it possible to obtain a photonic crystal having a stable shape and, thus, it is possible to select freely the material that is loaded in the photonic band-forming region or the region other than the photonic band-forming region. As a result, it is possible to increase the change in the refractive index in the photonic band-forming region, making it possible to obtain a satisfactory photonic crystal having a large change in the refractive index, as desired.
According to a second aspect of the present invention, there is provided a method of manufacturing a three dimensional structure, comprising a porous body and a plurality of regions formed in the porous body and loaded with a substance, an average period of a part of said plural regions loaded with said substance being 0.1 to 2 xcexcm to form a photonic band, comprising the steps of: loading a photosensitive material in the pore of the porous body; selectively exposing a predetermined region of the porous body loaded with said photosensitive material; and selectively removing the photosensitive material in the exposed portion or non-exposed portion of the porous body after the exposure.
In the method of the present invention for manufacturing a three dimensional structure, the region for forming a photonic band is supported by a porous body, making it possible to obtain a photonic crystal having a stable shape. Therefore, the periodic arrangement of the particular regions can be set freely. In addition, since it is possible to select freely the material that is loaded in the photonic band-forming region or the region other than the photonic band-forming region, it is possible to increase the change in the refractive index. It follows that a good photonic crystal can be formed at a low cost.
According to a third aspect of the present invention, there is provided a three dimensional structure, comprising a porous body, and a three dimensional wiring pattern formed by a conductive material loaded in the porous body, wherein said three dimensional wiring pattern includes at least three first layers each having a two dimensional wiring pattern and arranged in a direction perpendicular to the plane of the two dimensional wiring pattern, and at least two second layers interposed between two adjacent first layers and having joining portions for joining the two dimensional wiring pattern of the first wiring layer interposed between these two second layers.
In the three dimensional structure of the present invention, the three dimensional wiring pattern is formed of a conductive material loaded in a porous body having through-holes continuous in a three dimensional direction. Thus, since the wiring is supported by the porous body, a difficulty such as a peeling is unlikely to take place. Also, the present invention has made it possible to obtain a three dimensional structure having a three dimensional free shape and a steric wiring of a high density and adapted for use as a three dimensional wiring structure. In addition, since the porous body used in the present invention differs from the type of a plain weave of a fiber such as a mesh-like screen, it is possible to ensure stability in the shape and size of the three dimensional structure.
According to a fourth aspect of the present invention, there is provided a three dimensional structure, comprising a sheet-like first porous body having a two dimensional wiring pattern formed therein, and a sheet-like second porous body laminated on and made integral with said first porous body, wherein said second porous body has a joining portion connected to the two dimensional wiring pattern formed in said first porous body, and the first and second porous bodies have through-holes.
In the three dimensional structure of the present invention, the two dimensional wiring pattern is supported by the sheet-like first porous body and, thus, is unlikely to incur difficulties such as peeling. Also, the present invention has made it possible for the first time to form a fine wiring pattern and a via with a high dimensional accuracy. In addition, such a fine wiring pattern and a via can be formed easily. Such a three dimensional structure is adapted for use as a three dimensional wiring structure.
According to a fifth aspect of the present invention, there is provided a method of manufacturing a three dimensional structure, comprising the steps of selectively exposing a porous body to beam in a three dimensional wiring pattern having a plurality of two dimensional patterns in the incident direction of the beam, and selectively loading a conductive material or a precursor thereof in the pores in the exposed portion or non-exposed portion of the porous body after the exposure.
In the method of the present invention for manufacturing a three dimensional structure, a three dimensional exposure is employed, with the result that a positional deviation as in the conventional multi-layered wiring prepared by laminating a plurality of sheets does not take place at all. Further, a defective connection between the wiring and the via does not take place at all. It follows that the method of the present invention makes it possible to manufacture easily a fine and complex steric wiring structure.
Further, according to a sixth aspect of the present invention, there is provided a three dimensional structure, comprising a porous structure formed by a micro phase-separation structure and a conductive region formed by loading a conductive material in a predetermined region of the porous body, wherein said porous body is formed by removing at least one kind of the phase constituting a micro phase separation structure.
In the porous body formed by the micro phase separation structure included in the three dimensional structure of the present invention, the pores are formed homogeneously and regularly. In addition, the continuity of the pores is satisfactory. As a result, the dimensional stability is excellent and the conductance of the conductive region can be improved, compared with the case of using a conventional porous body prepared by, for example, elongation. Also, since it is possible to form pores of submicron order homogeneously, it is possible to form a fine wiring and a fine via. Further, since the width and thickness of the conductive region can be made uniform, the impedance characteristics are improved. What should also be noted is that, since the light scattering can be diminished in the exposure step, a fine pattern can be formed with a high accuracy.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.